The Dominator of Strings

Here you have a set of strings. A dominator is a string of the set dominating all strings else. The string $S$ is dominated by $T$ if $S$ is a substring of $T$.

The input contains several test cases and the first line provides the total number of cases.
For each test case, the first line contains an integer $N$ indicating the size of the set.
Each of the following $N$ lines describes a string of the set in lowercase.
The total length of strings in each case has the limit of $100000$.
The limit is 30MB for the input file.

For each test case, output a dominator if exist, or No if not.